In the traditional thermoplastic fiber melt spinning process, fibers of, for example poly(ethylene terephthalate) (PET), are spun and then subjected to a subsequent drawing process to impart desirable tensile properties to the fibers. The traditional spin-draw process, whether carried out in a two-step or as a continuous process, is energy and cost intensive due to the complexity of the operation and to the equipment involved. Nevertheless, high strength industrial fibers such as PET and nylon find widespread use in commerce and have resulted in the availability of numerous improved products including bias and radial tires, sewing thread, industrial fabric and the like.
Because of the widespread commercial use of industrial fibers, considerable effort has been directed toward providing fibers of improved properties. As a result of decades of research and development, there have been numerous processes proposed for producing high tenacity, high modulus fibers. However, many of these techniques have proven to be laboratory curiosities, limited to small scale batch procedures. Despite intensive effort, the properties of commercial fibers are still several orders of magnitude below theoretically possible values. For example, PET polymer has been reported to have a potential theoretical tenacity of 232 g/d, T. Ohta; Polym. Eng. Sci. 23, 697 (1983). But despite the decades of substantial research and development, current industrial PET yarns have a tenacity of about 9 g/d, a value far below the theoretical value.
During the past decade, efforts have been focused on high speed spinning of fibers. In Frankfort and Knox, U.S. Pat. No. 4,134,882, oriented crystalline PET fibers possessing good thermal stability and good dyeing properties were spun in a one-step process at take-up speeds of up to 7,000 m/min. Numerous other researchers have attempted to adapt the benefits of high speed melt spinning to produce various synthetic fibers including PET, polyamide 6, polyamide 66, and polyolefins such as polypropylene.
The high-speed melt spinning studies have resulted in the general recognition that concentrated deformation in the threadline, appearing as a neck like deformation, is generally correlated with the cooling rate and to the stress in the threadline. The stress is of primary importance since it is the main source of molecular orientation and of the subsequent structure development. The increased stress also results in a stress induced fiber crystallization. Although relatively high levels of stress have been obtained in fiber formation via ultra high-speed spinning employing spinning speeds of up to, for example, 12,000 m/min., fibers thus produced still possess poor mechanical properties due to insufficient time for the completion of structure development, to the development of a severe radial inhomogeneity of fiber structure and to the formation of voids in the sheath portion of the fiber.
The use of liquids for an in-line coupled spin-draw process was proposed nearly three decades ago in U.S. Pat. No. 3,002,804 to Kilian. In this process, melt spun filaments were quenched by cooling air or by a liquid drag bath to at least 50.degree. C., and preferably 100.degree. C., below the melting point of the filaments prior to or concurrent with the entry of the filaments into the liquid drag bath. The liquid drag bath was positioned at a distance of up to twenty-four inches and preferably four to six inches, below the face of the spinneret. The liquid drag bath was provided by a container having a restricted orifice in its bottom wall or by a long tube positioned vertically in the path of the filament. The liquid drag bath was used at ambient temperature or heated to a temperature of 80.degree.-90.degree. C. up to 94.degree. C. The maximum tenacity of filaments reported was 7.7 g/d employing a liquid drag bath of 10 feet in length positioned 4 inches below the face of the spinneret using a wind-up speed of 3,000 yards per minute (2,750 meters per minute).
A process similar to the Kilian process was proposed at about the same time in Canadian Patent No. 670,932 to Thompson and Marshall (1963). In the case of this process, a water bath at a temperature above the second order transition temperature of the spun filaments was positioned at a location near the spinneret such that the filaments entered the high temperature water without being substantially heated or cooled. The filament was passed over a guide at the bottom of the bath and was taken back to the surface of the bath over another guide and a wind-up bobbin. The maximum wind-up speed was maintained preferably below 3,000 yards per minute (2,750 meter per minute). The maximum tenacity of filaments thus produced was 3.4 g/d at a path of 270 cm. in water bath at 88.degree. C.
A liquid quenching process was proposed in U.S. Pat. No. 4,932,662 to Kurita et al. In this process, a liquid quenching tube maintained at a temperature of less than or equal to 50.degree. C. was positioned at a distance from the spinneret where the filament was not solidified. A fast quenching effect occurred in the filament to suppress crystallization. In addition to the quenching apparatus, a draw-heating zone was added to the threadline subsequent to the quenching step. In this process, filaments used for the subsequent drawing and heat treatment had a high differential in molecular orientation between the yarn surface and center, ca. 5.times.10.sup.-3 and preferably 10.times.10.sup.-3. After the drawing and heat treatment, the spun filaments also exhibited a substantial radial variation of birefringence ranging from 7.0.times.10.sup.-3 to 14.times.10.sup.-3. The maximum tenacity of filaments was reported to be 11.31 g/d at 25 cm. of the quenching tube and with a 1.31 draw ratio using steam at 245.degree. C. between a set of draw rolls.
U.S. Pat. No. 4,909,976 to Cuculo et al. discloses an advantageous process for optimizing fiber structure (orientation and crystallization) development along the threadline during high speed melt spinning. This process employs a zone cooling and zone heating technique to alter the temperature profile of the moving threadline to enhance structure formation. Take-up stress remained almost unchanged as compared with that of conventional melt spinning.
Despite the decades of intensive research, commercial processes for producing high strength, high modulus fibers from commonly available polymers such as PET are limited to in-line or two-step spin draw processes using mechanical drawing apparatus. Moreover, fibers possessing desirable properties of high strength, high modulus, high orientation and which are of high radial uniformity are nevertheless still far below potentially obtainably values.